The present invention relates in general to coils useful in nuclear magnetic resonance (NMR) apparatus, and more specifically to a balanced configuration for shelf-shielded gradient coils.
Magnetic Resonance Imaging (MRI) systems are currently employed in forming tomographic images of the internal human anatomy. In such systems, a patient is placed in a static magnetic field and is subjected to radio-frequency electromagnetic pulses to excite nuclear spins. The magnetic resonance of the atomic nuclei of the patient is detected with a receiving coil to provide information from which an image of that portion of the patient containing these nuclei may be formed. The magentic field possesses gradients which are pulsed on during each detection sequence so that the position of resonating nuclei can be determined. These same phenomena are employed in magnetic resonance spectroscopy for analyzing properties and structures of substances.
A shielded type of gradient coil is known from U.S. patent application Ser. No. 826,650, now U.S. Pat. No. 4,737,716, filed Feb. 6, 1986, which is hereby incorporated by reference. A shielded gradient coil is desired to reduce various interactions of the gradient fields with other structure such as the main field magnet. The interactions include field spatial and temporal distortions, energy dissipation and annoying audible sound.
In the preferred embodiment of the previous invention, concentric inner and outer coils are employed for each gradient axis. The inner and outer coils for each axis are connected in series. The surface current distributions of each coil set result in a gradient magnetic field inside the coil set and a substantially zero field outside the coil set.
Typical MRI systems employ gradient magnetic fields along three orthogonal axes in the x, y and z directions. The z-axis is usually defined as coinciding with the direction of the main static magnetic field (which is usually along the axis of the main cylindrical magnet) and the x- and y-axes are perpendicular to the static field. The gradient coil set for each axis has its own respective gradient amplifier which is under control of the imaging system.
The coil sets must be operated in close proximity to each other. However, it has been found that problems can occur during pulsed operations of the gradient coils resulting from interactions between separate coil sets. For example, the gradient amplifiers may become unstable.
Accordingly, it is a principal object of the present invention to improve the operation of gradient apparatus in MRI systems.
It is another object to reduce or compensate for interactions between coil sets in close proximity.
It is yet another object of the invention to avoid unstable operation of gradient amplifiers used in conjunction with self-shielded gradient coils.